This year's flu shot protects against three flu strains, two of which were not in last year's vaccine.

All Americans over 6 months old should get their flu shot this year, health officials urged Sept. 27. While last year's flu season was relatively mild, the illness can be unpredictable, they said.

"People cannot become complacent this year," Dr. Howard Koh, assistant secretary for health at the U.S. Department of Health and Human Services, said today at a news conference about influenza vaccinations. "When it comes to flu, we can't look to the past to predict the future. Don't take the risk … get your vaccination," Koh said.

There are expected to be 135 million doses of flu vaccine available over the coming year, and more than 85 million have been distributed as of mid-September, said Dr. Daniel Jernigan, deputy director of the influenza division at the Centers for Disease Control and Prevention (CDC).

Because flu strains circulating around the world are constantly changing, a new flu vaccine is made every year. This year's flu shot protects against three flu strains, two of which were not in last year's vaccine.

"Getting vaccinated last year won't cover you for these new strains," Jernigan said. The best time to get vaccinated is now — before flu season starts in October, Jernigan said.

So far, the main viruses in circulation this year match the ones in the vaccine, Jernigan said. Studies show that when this happens, people who get vaccinated are 50 to 60 times less likely to get the flu compared with unvaccinated individuals.

During the 2011 to 2012 flu season, 128 million Americans, or 42 percent of the population, received a flu shot, according to the CDC. That's very similar to the 43 percent that were vaccinated the year prior, Koh said.

Among children, 52 percent were vaccinated last year. Vaccination rates are highest among young children ages 6 to 23 months, at 75 percent, but rates dropped to 34 percent among adolescents ages 13 to 17, Koh said.

Nearly half of pregnant women, or 47 percent, were vaccinated last year. Vaccination during pregnancy is important because pregnant women are particularly susceptible to complications from the illness.

In fact, pregnant women are five times more likely to become very ill if they get the flu compared with women who are not pregnant, said Dr. Laura Riley, director of labor and delivery at Massachusetts General Hospital in Boston. The flu shot can be given at any time in pregnancy, and provides protection to the unborn child as well, Riley said.

It is estimated that flu vaccines last year prevented 5 million cases of influenza, and 40,000 hospitalizations, Koh said.

The vaccine is available in three forms, the traditional vaccine, a nasal spray and a high-dose injection for people ages 65 years and older.

Flu vaccinations are becoming increasingly available outside traditional medical settings, in locations such as pharmacies, schools and places of employment. Last year, more than 20 million doses of flu vaccine were administered by pharmacists, said Mitchel Rothholz, chief strategy officer for the American Pharmacists Association.

Flu vaccinations are recommended for everyone ages 6 months and older, a recommendation that has been in place since 2010.